Michael is 21 years older than Tiffany. Four years ago, Michael was 4 times as old as Tiffany. How old is Tiffany now?
Explanation: We can use the given information to write down two equations that describe the ages of Michael and Tiffany. Let Michael's current age be $m$ and Tiffany's current age be $t$ The information in the first sentence can be expressed in the following equation: $m = t + 21$ Four years ago, Michael was $m - 4$ years old, and Tiffany was $t - 4$ years old. The information in the second sentence can be expressed in the following equation: $m - 4 = 4(t - 4)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $t$ , it might be easiest to use our first equation for $m$ and substitute it into our second equation. Our first equation is: $m = t + 21$ . Substituting this into our second equation, we get the equation: $(t + 21)$ $-$ $4 = 4(t - 4)$ which combines the information about $t$ from both of our original equations. Simplifying both sides of this equation, we get: $t + 17 = 4 t - 16$ Solving for $t$ , we get: $3 t = 33$ $t = 11$.